1. Field of the Invention
The present invention relates to a camera having a photographing optical system and a finder optical system separate therefrom, and in particular, it relates to a finder optical system thereof. The present invention also relates to a control method of a camera having a real-image finder.
2. Description of the Related Art
Recent cameras in general, including single lens reflex cameras and compact cameras, are being provided with an automatic focusing function. In a single lens reflex camera, since the focus state of an image formed on a focusing plate within a view finder changes during the focusing operation of the photographing lens, it is possible for a photographer to judge whether or not the photographing lens is focused on an object by the automatic focusing operation while viewing the image through the view finder.
In a camera in which the finder system is provided separately from the photographing lens system, the finder system constitutes a real image telescope or a virtual image telescope, and hence, in general, no focusing mechanism is provided since objects at different object distances can be viewed in an in-focus state due to the human eyes' control ability. Instead, an indicator for lighting a lamp when the object distance is detected is provided in or in the vicinity of the view finder, so that a photographer judges that the object is in focus when the lamp is lit and actuates a shutter.
However, in cameras other than single lens reflex cameras, it is impossible for a photographer to confirm, lo prior to the photographing operation, whether or not the photographing lens can be correctly focused on an object by the automatic focusing operation. Consequently, if the automatic focusing operation is carried out without correctly focusing the object being focused on in the distance detection area of the view finder, or if a malfunction of the automatic focusing system occurs for an object such as a scene through a glass or flames, the object distance of which cannot be easily detected by the distance detection system, the object is not in focus.
Namely, since the confirmation indication such as the lamp within or in the vicinity of the view finder is adapted only to confirm whether the object in the object distance detection area is in focus, if the photographer fails to appropriately operate or the mal-function of the distance detection system occurs, even if the photographer takes a picture after he or she confirms that the lamp is lit, the picture will be out of focus.
Note that the focus state in the view finder can be also confirmed by a focusing plate, as used in a single lens reflex camera, which is provided with fine projections and depressions and is located at the image forming position (focal plane) in which an object image is formed by an objective lens. However, finder optical system for recent compact cameras are very small, so that the focal length of the eyepiece is in the range of approximately 20 to 30 mm, and the ocular magnification thereof amounts to 250/f.sub.e ; wherein f.sub.e is the focal length of the eyepiece lens. This value is extremely high in comparison with the magnification (3 to 5) of an eyepiece of a conventional 35 mm single lens reflex camera. Therefore, if a focusing plate is provided in a finder optical system of a compact camera whose magnification is extremely high, the visual field of a finder looks poorly finished if the pitch of the fine projections and depressions of the focusing plate is not small enough in comparison with that of the focusing plate in a single lens reflex camera. If the pitch of the projections and depressions is small, the diffusibility is reduced and consequently it is difficult to recognize the amount of defocus (focal-shift) of the finder image.